How Much Warning?

The amount of warning time possible in an earthquake depends on your distance from the rupturing fault, the earthquake's depth, and the speed of the earthquake warning system.

The farther you are from an earthquake, the less it will affect you, and the more warning time you can get. Perhaps 60 seconds of warning is possible for very large, far away earthquakes, but these are very unlikely.

Unfortunately, California cities are generally located near, or on top of, dangerous faults. Moderate to large earthquakes (magnitude 6.0 and above) on these faults are far more likely: 10 times more likely. 1994 Northridge (which killed 67 people) and 1989 Loma Prieta (which killed 62 people) are examples of urban earthquakes.

There will be less warning possible for these urban earthquakes, so an earthquake warning system must be fast enough to provide every second of warning that is possible.

The warning must arrive before the serious shock waves for it to be useful. This may seem obvious, but most earthquake warning systems fail to do this within 8-15 miles from the epicenter. There are a lot of people in those 50-175 square miles.

The interactive tools below let you explore the factors that determine how much warning time is possible. These apply to ALL earthquake warning technologies and systems.

How Much Warning Time is Possible?

Earthquakes and Shock Waves

Two types of shock waves are produced by all earthquakes: P-wavesPrimary, or P, waves are the fastest moving waves produced by an earthquake and are first to arrive. They do not carry much energy and cause only slight shaking. and S-wavesSecondary, or S, waves are the first waves to arrive from an earthquake that are large enough to cause damage. They are slower than P-waves but cause much greater shaking.. P-waves travel faster than S-waves, but they have very little energy and don't cause damage. Damage starts with the arrival of the S-waves.

S-waves travel very quickly; over 2 miles/second (3.6 km/second). If you're close to the epicenter, you'll feel them almost immediately. Farther away, the delay can be dozens of seconds.

The farther the shock waves travel, the weaker they become. A magnitude 6.0 will feel like a magnitude 5.0 if you're 60 miles (100 km) away.

INTERACTYou can use the tool to explore how S-wave arrival time depends on the earthquake's depth and how far away you are.

Tell me more: Tell me more »

An earthquake is a ruptureA slip along a fault. The length of the slip is related to the earthquake's magnitude. A large earthquake can cause a rupture along hundreds of kilometers. on a fault.

Earthquake warning works by detecting the shock waves produced by the earthquake. There are two types: P-wavesPrimary, or P, waves are the fastest moving waves produced by an earthquake and are first to arrive. They do not carry much energy and cause only slight shaking. and S-wavesSecondary, or S, waves are the first waves to arrive from an earthquake that are large enough to cause damage. They are slower than P-waves but cause much greater shaking..

The first to arrive are the P-wavesPrimary, or P, waves are the fastest moving waves produced by an earthquake and are first to arrive. They do not carry much energy and cause only slight shaking., which travel fastest: about 3.8 miles/second (6.2 km/second). The S-wavesSecondary, or S, waves are the first waves to arrive from an earthquake that are large enough to cause damage. They are slower than P-waves but cause much greater shaking. take longer to arrive since they travel more slowly: about 2.2 miles/second (3.6 km/second).

The rupture begins underground (typically 5 miles deep in California). This location is called the hypocenter, or the nucleation point.

How soon you experience an earthquake depends on your distance from the hypocenter: it depends on the earthquake's depth and on how far you are from the ruptureA slip along a fault. The length of the slip is related to the earthquake's magnitude. A large earthquake can cause a rupture along hundreds of kilometers..

Time it takes for the shockwaves to reach you

distance from earthquake: 0 km
depth of earthquake: 0 km
The average depth for earthquakes is 8 km
km miles

How Long Will it Take to Receive a Warning?

What's important is when you receive the warning, not when it is sent. Let's look at each of the factors affecting how quickly the warning can be delivered.

Detect the Start of the Earthquake

The earthquake begins underground, so the P-wavesPrimary, or P, waves are the fastest moving waves produced by an earthquake and are first to arrive. They do not carry much energy and cause only slight shaking. must first reach the surface before there can be any knowledge that the earthquake has begun.

Analyze the P-waves

The P-wavesPrimary, or P, waves are the fastest moving waves produced by an earthquake and are first to arrive. They do not carry much energy and cause only slight shaking. must be evaluated to recognize an earthquake (rather than a truck) and estimate how big it will be (to ignore the tiny ones). Analyzing P-waves takes time. Many techniques require 3-4 seconds to accurately evaluate the earthquake. Ours takes 0.5 second.

Distribute the Warning

Distribution of the warning via the Internet can be fast, less than 0.1 second if optimized. Distribution via wireless is slower, especially during peak traffic times. Delays are often 5 seconds and longer. Messages sent via Wireless Emergency Alerts (WEA) or the Emergency Alert System (EAS) can take 30 seconds to arrive.

We manage our communications system to keep delays to 0.1s.

INTERACTYou can use the tool to explore the time it takes to receive a warning. Selecting the SWS option, shows the timings for our system.

SWS = Seismic Warning Systems
other = other EQW systems
cell = cellphone notification
EAS = Emergency Alert System (WEA is part of EAS)
manual = select arbitrary values

Time it takes for you to receive a warning

depth of earthquake: 0 km
The average depth for earthquakes is 8 km
P-wave analysis time: 0 seconds
SWS other manual
communications time: 0 seconds
SWS cell EAS manual

Putting it all Together: How Much Warning Will You Get?

The amount of warning time depends on the earthquake and the performance of the earthquake warning system.

The warning must arrive before the serious shock waves for it to be useful. This may seem obvious, but most earthquake warning systems fail to do this within 8-15 miles from the epicenter. There are a lot of people in those 50-175 square miles.

Our system is engineered to detect the earthquake and deliver a warning faster than any other system.

INTERACTYou can use the tool to see how much warning time is possible. Note that near the epicenter, slow systems provide a warning AFTER the S-waves arrive. In California, the most likely earthquakes are in urban locations, where people will be close by.

Tell me more: Science »
Real World Warning Times

Many statements are made in the press about how much warning time earthquake warning will be able to deliver, often suggesting minutes of warning is likely. This is misleading at best.

The chart below was published in 2006 By Dr. Richard Allen of UC Berkeley, who is a chief contributor to the USGS earthquake warning system. It shows the probability of warning times in San Francisco. It was created by looking at all the earthquake scenarios that threaten the city and calculating the warning time and shaking intensity for each scenario.

For 99% of earthquakes that will cause damaging ground motions in the SF Bay Area, only 25 seconds or less warning is possible. Notice also that there is a significant probability that the system described here will produce a warning many seconds after the shaking begins. That's the Blind ZoneA region, centered on the epicenter, where no warning is provided before serious ground motions arrive. A few seconds of delay producing a warning can produce a Blind Zone covering 400 square miles..

Warning times in other urban areas in California will be similar, since they are all located near dangerous faults.

Our earthquake warning system has no Blind Zone. This means that we'll deliver warning before the S-wave arrives for all likely earthquakes.

Warning Times for SF Bay
CC-Attrib/Richard Allen, 2006

The amount of warning time you'll get

distance from earthquake: 0 km
depth of earthquake: 0 km
The average depth for earthquakes is 8 km
P-wave analysis time: 0 seconds
SWS other manual
communications time: 0 seconds
SWS cell EAS manual

For More Information